Apparatus and method for wellbore resistivity measurements in oil-based muds using capacitive coupling
AssigneeBaker Hughes Incorporated
InventorsLeonty Abraham Tabarovsky, Albert Alexy
Life of this patent
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Abstract
A resistivity device for downhole use with a nonconducting mud injects a current at a frequency sufficiently high to pass capacitively through the mud and a mud cake into the formation. The frequency is further selected so that the impedance due to the dielectric constant of the formation is small compared to the formation resistivity. Dual frequency measurements may be used to further minimize the effects of the mud and mud cake resistivity. When multiple frequency measurements are used, frequency focusing may be used to get an estimate of formation resistivity that is independent of near borehole effects.
Claims14
An apparatus for use in a borehole for obtaining a resistivity parameter of an earth formation penetrated by the borehole, the borehole having a substantially nonconducting fluid having a dielectric constant therein, the apparatus comprising:
- (a) a downhole tool including at least one measure electrode coupled to the earth formation through said nonconducting fluid, said at least one measure electrode conveying a measure current from a source thereof into the formation. said measure current having a frequency selected to make an impedance caused by said dielectric constant to be substantially less than a resistivity of said nonconductive fluid, said frequency being further selected to make an impedance caused by a dielectric constant of said formation to be substantially less than a resistivity of said formation;
- (b) a device responsive to least one of (I) the current in the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter.
A method of obtaining a resistivity parameter of an earth formation penetrated by a borehole having a substantially nonconducting fluid therein, the method comprising:
- (a) conveying a measure current through a measure electrode into the earth formation by capacitive coupling through the nonconducting fluid, said measure current having a first frequency selected to make an impedance caused by said dielectric constant to be substantially less than a resistivity of said nonconductive fluid, and an impedance caused by a dielectric constant of said formation to be substantially less than a resistivity of said formation;
- and
- (b) determining at least one of (I) the current at the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter.
A method of obtaining a resistivity parameter of an earth formation penetrated by a borehole having a substantially nonconducting fluid therein, the method comprising:
- (a) conveying a measure current through a measure electrode into the earth formation by capacitive coupling through the nonconducting fluid, said measure current having a first frequency selected to make an impedance caused by said dielectric constant to be substantially less than a resistivity of said nonconductive fluid;
- (b) repeating (a) at a second frequency,
- (c) determining at least one of (I) the current at the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter at said first frequency and said second frequency;
- and
- (d) determining from said measure signals at said first and second frequencies said resistivity parameter.
A method of obtaining a resistivity parameter of an earth formation penetrated by a borehole having a substantially nonconducting fluid having a dielectric constant therein, the method comprising:
- (a) conveying a measure current through a measure electrode into the earth formation by capacitive coupling through the nonconducting fluid at a plurality of frequencies, each of said plurality of frequencies selected to make an impedance caused by said dielectric constant to be substantially less than a resistivity of said nonconductive fluid;
- (b) determining at least one of (I) the current at the at least one measure electrode, and, (ii) a voltage of the at least one measure electrode for producing a measure signal representative of the resistivity parameter at each of said plurality of frequencies;
- and
- (c) frequency focusing apparent conductivities derived from said measure signal at each of said plurality of frequencies and obtaining said resistivity parameter.
Dependencies are inferred from the claim text.